Bond lengths Cope rearrangements

Step through the sequence of stmctures depicting Cope rearrangement of 1,5-hexadiene. Plot energy (vertical axis) vs. the length of either the carbon-carbon bond being formed or that being broken (horizontal axis). Locate the transition state. Measure all CC bond distances at the transition state, and draw a structural formula for it... 

A number of observations can be made from inspection of Table 30.2. First, at Rq, the interallylic bond length in the TS for the parent Cope rearrangement, a phenyl group at C2 provides 1.7 kcal/mol of stabilization, which is slightly more than twice that provided... 

Table 30.2 shows that at/f = 1.599 A the C2 phenyl group in 2-phenyl-1,5-hexadiene provides a net stabilization of = —2.9 kcal/mol, and atR = 2.218 A the Cl and C3 phenyl groups in 1,3-diphenyl-1,5-pentadiene provide a net stabilization of = —2.0 kcal/mol. However, the TS for Cope rearrangement of 1,3,5-triphenyl-1,5-pentadiene occurs with interaUyhc bond lengths of about R = 2.110 A (Table 30.1). At this TS geometry, neither the phenyl groups at Cl and C3 nor the phenyl group C5 provides as much stabilization as these phenyl groups furnish in the TSs for the Cope rearrangements of, respectively, 1,3-diphenyl-1,5-pentadiene at / = 2.218 A and...

Table 2 Calculated C-C bond lengths in A for the Cope rearrangement of Figure 4. E, I, and P denote the educt (left in the figure), intermediate (middle), and the product (right), whereas TS(E-I) and TS(I-P) are the transition states for the two reaction parts. Finally, both singlet and triplet structures are considered for the intermediate, and Cl and hybrid specify the method of calculation. All results are from ref 31...

Table 19 Calculated Transition State Bond Lengths and Activation Barriers for Cope Rearrangement and Claisen Rearrangement. Refer to Figure 4 for the Structure of Transition State and Notations of Bond Lengths ...

Because of the dispute over these questions, a large number of different methods have been used for the investigation of the Cope rearrangement the results from some of the most important aie summarized in Table 9. For the C2 -symmetric chair transition structure, most methods predict an aromatic character with C1-C6 bond lengths ranging from 1.93 to 2.19 A. The aromatic character of the transition structures was also confirmed by calculations of the magnetic properties of the transition structure. In contrast, the perturbation... 

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